


Differential

by Island_of_Reil



Category: Shingeki no Kyojin | Attack on Titan
Genre: Biting, Dominance, Floor Sex, Light Sadism, M/M, Masochism, Rough Sex, Size Difference, Spanking, Submission, Wall Sex, dom!armin, taller!Armin
Language: English
Status: Completed
Published: 2014-07-27
Updated: 2014-07-27
Packaged: 2018-02-10 17:05:32
Rating: Explicit
Warnings: Creator Chose Not To Use Archive Warnings
Chapters: 1
Words: 4,278
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/2032965
Author URL: https://archiveofourown.org/users/Island_of_Reil/pseuds/Island_of_Reil
Summary: <blockquote class="userstuff">
              <p>At seventeen, Armin is two heads taller than Eren. And in charge. At least for the moment.</p>
            </blockquote>





	Differential

**Author's Note:**

> [Kinkmeme prompt.](http://snkkink.dreamwidth.org/13546.html?thread=8531178#cmt8531178)

_Thud._ “Ow!”

Armin screws his eyes shut and rubs the crown of his head, which he’s just smacked against the lintel of Eren’s bedroom door. It’s the third time this week he’s banged his head on a doorframe. When he opens his eyes again they narrow instantly. “Don’t laugh at me, Eren!”

“I’m not laughing at you!” Eren lies, trying to keep his lips from quirking.

He does feel bad about it. It’s not as though Armin _asked_ to grow two heads taller than Eren over the last three months — taller than Commander Smith was, taller even than Bertholdt was. And, anyway, he’s still built like a rail. Eren still has the broader shoulders, the heavier musculature, the deeper voice. Even if Armin’s has finally cracked and begun something of a descent.

Still. Eren somehow thought he’d always be taller than Armin, always able to loom protectively over him. When he thinks about his dad — to the extent that he can without passing out — he can’t remember how tall he was. Can’t even remember how much taller he was than Eren’s mom. You see the world differently at ten than you do at seventeen.

He remembers his dad’s beard. _That’s_ come in on him, at least. Armin’s face is more angular than it used to be, but nothing grows on it except pale hairs no coarser than duckling down. Eren tries to remember that when he’s shaving, which he has to do every day now, because to be honest he looks better clean-shaven. He’s clumsy with a razor and the cuts hurt, even if they close up right away with little puffs of steam.

“Liar,” Armin says. He’s stopped rubbing the top of his head. He wears his hair shorter these days, on the spiky side, like Jean’s; just now it’s a tousled mess. His eyes are still narrowed, but they’re also a little darker. And then Eren’s back is hitting the wall with a thump and his wrists are caught up and pinned above his head. Armin is faster than he used to be, as if he’s changing into one of the lean, long racehounds that only royalty and nobility may breed and keep.

Eren’s still stronger than he is and could push him away easily. He doesn’t. It’s as though being thrown against the wall’s jostled all the blood in his body into his groin. Armin looking down at him with pupils flowering black against the blue, and a cunning smile tilting his lips, isn’t helping.

Armin ducks his head to kiss him. Eren’s especially not used to _that_ sequence of movements yet, and it irks him even as he anticipates the taste of Armin’s tongue in his own mouth. He raises his head and opens his lips and takes Armin in, lets his tongue sweep and suck and flutter around his own. He tries to embrace Armin, forgetting that his hands aren’t free. Armin tightens his grip on Eren’s wrists and flattens them against the wall above his head.

“Can’t I even put my arms around you?” Eren complains as he pulls his mouth away. The words come out whiny, to his chagrin.

Armin’s grinning down at him now. “No,” he says, amused, and he leans forward and grinds against Eren. He’s gotten bigger in other ways, too. Not hugely so, all things considered, but there’s no fat for it to disappear into, not enough muscle to compete with the feel of it pressing against Eren’s belly and lower ribcage.

Eren closes his eyes in both arousal and exasperation. “Can we move this party to the bed, at least? So you can rub it against something a little more sensitive?”

“You were laughing at me—”

Eren’s eyes fly open angrily. “I _wasn’t!_ ”

“—and you lied about it, and you’re lying about it again. You’re a terrible liar, by the way. Why should I give you what you want, Eren?”

Armin drops his hands to Eren’s armpits. He’s still not especially strong, not for the Survey Corps. But he’s strong enough to — with a strained grunt, admittedly — hoist Eren off his feet about a quarter-meter and keep him pinned against the wall with his hands on Eren’s shoulders and his hips against Eren’s. Eren is too startled to do anything but gape into his face, now only a few centimeters from his own, until Armin drops his head into the crook of Eren’s neck and bites down. Hard.

“Aagh!” Trying to jerk away from Armin doesn’t really work in this position. Armin raises his head again and kisses Eren hard, and Eren tastes blood on Armin’s lips, his own lips. He hears the sizzle of steam at the join of his neck and shoulder as the bite wound begins to heal. In twenty minutes there’ll be no sign there that Armin went all vampire on him.

“Isn’t that… unsanitary?” he huffs against Armin’s mouth.

“Commander Hange says … you can’t contract or pass on … blood poisoning,” Armin murmurs back.

“You… _asked_ her about this?” Eren hisses incredulously before Armin’s tongue is weighing down his own again.

The kiss breaks with a wet pop as Armin replies, “No… came up on its own … gave me ideas.”

The next time he pulls away Eren mutters, “You and ‘ideas’ … fucking dangerous combination,” and Armin laughs low and surprisingly deep as he reunites their mouths.

His hard-on is pressing more or less right into Eren’s, but with his feet hanging in mid-air Eren has no leverage with which to grind against it. It’s more of a tease than having it pressing into his lower ribs was. “Come _on,_ Armin,” he protests the next time his mouth is free. “We can’t really do anything in this position.”

“Mm, yeah, you’re right.” Armin grunts again as he lowers Eren to the floor. He’s not quite strong enough to manage a slow, gentle drop; the floor thuds hard against the soles of Eren’s boots and the blow judders through him, knocking the breath out of him as he squeezes his eyes shut and grits his teeth. “Sorry,” Armin wheezes, then catches his breath and stoops a little and starts attacking the gear strap buckles around Eren’s waist.

“Oh, _fuck,_ ” Eren gasps, flattening himself against the wall with his hands behind him, arching his hips outward. He doesn’t know if Armin plans to jerk him off or suck him off or rub their cocks together, but who cares, it’s all good. He makes a small noise of surprise when, instead of merely taking Eren’s cock out, Armin shoves Eren’s trousers and underwear down around his knees.

“Get those off,” Armin orders, turning around and heading for the night table next to Eren’s bed.

Eren just stands there and blinks at the long, shirt-clad stretch of Armin’s retreating back. “What—”

Armin pulls open the drawer and takes out a small bottle, then shuts the drawer and turns around again. “Eren.” His voice is suddenly stern as he tucks the bottle into a trouser pocket. “I said to take your trousers and underwear off. If you’re going to be difficult I’m going to make you pay for it.”

All of a sudden Eren _gets it,_ and his face turns bright red. “So now that you’re taller than I am you’re in charge all of a sudden?” Eren demands. And, goddamn, does _that_ ever come out petulant.

Armin’s brows lift. Armin is better with words than just about anyone Eren’s ever met, except maybe Commander Smith, but with only his eyebrows he’s made Eren feel much more childish than with anything he could have said.

“Not for good,” Armin says. His eyes are about the same, dilated yet calculating, but they fix Eren’s in the same way his hands pinned Eren’s to the wall. “Just right now, though? Yes, I am. Take it or leave it, Eren.”

Eren stares at him, both his throat and his trousers tightening, and his face hot enough to cook on. Then he lowers his head and lifts his left foot and pulls off his boot.

“Good boy,” Armin whispers. Eren doesn’t dare look at him as he pulls off the other boot, then sets the both of them aside, then shoves his trousers and underwear all the way down to his ankles and steps out of them. As he straightens up, the head of his cock bumps softly against his belly.

“Turn around, Eren. Hands flat against the wall. Head down. Feet apart. Ass out.”

His _everything_ feels hot enough to cook on now, especially with those last two words, but he obeys. The still-fresh plaster — Trost HQ was rebuilt just last year — is cool and bumpy against his palms, the lacquered pine floorboards smooth under his soles as he plants them wide apart.

He hears and feels Armin come up behind him. One arm goes around him, fingers pulling on his left nipple through his shirt, rubbing the coarse linen weave into the erect nub of flesh. Eren gasps. He tries to grind backward against the cock jabbing into the small of his back. Then there’s a sudden sharp smack of hand on flesh, and his right buttock is stinging.

“Hold still. I didn’t give you permission to do that, did I?”

“Asshole,” Eren mutters. One of his favorite words, but he’s never applied it to Armin before. Armin slaps his ass again, the exact same spot as before, and Eren yelps. It hurts, but it hurts in a way that makes wet heat with a sharp edge to it curl in his belly.

“You like that,” Armin says, half accusing and half observing.

“No,” Eren lies, husky-voiced. This time the force of Armin’s hand on the same spot drives him forward against Armin’s left arm. He cries out loudly now, a choppy, shuddering yell, as his ass cheek burns and his cock twitches violently. Armin’s breath is hot against his nape as he reaches down and curls his right hand around Eren’s cock.

 _“Don’t. Lie. To. Me.”_ Armin grinds out each word discretely from between his teeth and punctuates each with a squeeze. He’s exerting enough pressure on Eren’s cock that it hurts. But it doesn’t hurt as much as it feels weirdly good. Eren prays that Armin doesn’t ask him if he likes that, too, because he’s afraid Armin will squeeze tighter. That he’ll like it even better when Armin squeezes tighter.

“Y-yeah,” he finally gasps, tilting his head back, feeling the bone of Armin’s shoulder hard under the back of his skull. “I liked that.”

Armin’s erection twitches against his back through Armin’s trousers and Eren’s shirt. Armin himself doesn’t speak or move for a moment. Then he drops his left arm from around Eren’s chest and his right hand from around Eren’s cock. Eren bites his lip so as not to whimper, both because Armin isn’t touching his cock anymore and also because he has a good idea what’s coming next.

This time Armin slaps his left ass cheek, and the blow is much harder than any of the previous three. Eren bucks forward with the impact and stumbles; his left hand trembles against the wall, the palm half-rising off the plaster. 

“I told you to keep your hands flat against the wall, Eren!” Armin says sharply. His hand lands in the same spot again with just as much force, and this time Eren whines around the teeth in his lower lip.

“I- I’m sorry,” Eren stammers, setting his palms back in place. He notes that Armin didn’t call him out for tilting his head back against Armin’s shoulder earlier, but all the same Eren bows it forward as far as he can.

The next thirty seconds are punctuated by hard slaps all over his ass and the upper backs of his thighs, Armin making a point of revisiting spots he’s struck before. After ten seconds Eren is mewling through clenched teeth. After twenty he’s wondering how the hell he could stoically take beatings from Annie Leonhardt and Captain Levi and dozens of titans, yet _this_ is turning him into a sobbing, teary mess. He feels like there can’t be any intact skin left on his ass, but he also can’t remember the last time he was this hard. He’s started to wail loudly when Armin stops, and Eren subsides into hitching gasps.

“I wish you could see your own ass, Eren.” Armin’s voice is kind of breathy, like he’s just read something amazing in his grandfather’s old book. “It’s bright, bright red.” Eren feels his hand stroking gently over it, smoothing it, and it hurts a little but it feels strangely good, comforting and arousing at the same time. “And the skin is so hot. Almost as hot as when you shift back.”

Armin’s stroking fingers slip downward between Eren’s thighs, trail up the back of his sac. Eren whimpers a little, partly out of pleasure, partly because he hopes the aforementioned dangerous combination of Armin and ideas doesn’t mean Armin’s going to hit him _there_ next. 

But Armin’s touch remains light, very light, and slowly it sweeps up into the cleft of Eren’s ass. Eren tenses a little. Usually they don’t fuck, they do other things instead, but he’s fucked Armin on a few occasions. Armin’s never returned the favor before, never even fingered Eren, actually.

“Mmph. Dry,” Armin whispers. His touch vanishes. Eren hears the rustle of fabric, and then the distinctly wet pop of a tiny cork. There’s another wet noise, faint and indistinct, then more rustling. Eren shivers and sucks in his breath as he feels fingers part his buttocks and one slick fingertip stroke over his hole.

“Do you like that?” The whisper is coming from about the level of his waist. He realizes that Armin is kneeling or crouching behind him.

“...yeah,” Eren manages to utter, his voice cracking. The fierce sting of Armin’s hand has dulled into a low-burning ache all over his ass and thighs. Something about how it contrasts with the very soft, well-oiled caresses at his opening is making his cock throb with a force that feels like it could lift him off his feet if he let it. He wants to drop a hand from the wall to palm himself with but he’s terrified that Armin will slap his ass again. He’s even more terrified that Armin will just stand up and leave because Eren disobeyed him.

“Try to relax,” Armin says, quietly and soothingly, but there’s authority there too. “You already know this shouldn’t hurt. But if it does, you’re going to tell me, Eren.” It’s not a question.

Eren nods vigorously. He’s been on the other side of this equation before, as Armin just implied. His need to hear those words has nothing to do with the literal information they convey.

Armin begins to insinuate the same finger into Eren. More than anything else, it feels odd, having something up there. Not pleasant or unpleasant, just odd. Armin slowly works it in all the way up to the first knuckle, then leaves it in place, not moving it at all. Eren can hear him begin to breathe a little more quickly.

“You’re so _hot_ inside.” Armin’s words are all breath, all wonder. “And _tight._ ”

Eren’s face is hot again, too. He’s said those same things to Armin, lots of times. No one’s ever said them to Eren before. It makes him feel keenly aroused and weirdly vulnerable and okay with being weirdly vulnerable, all at the same time. It’s Armin who’s saying these things to him. Armin might get a little rough with him, but Armin would never truly hurt him.

The finger slides out, then in, twice more. The third time, a second fingertip begins to push in alongside it. Armin works both into Eren gently and slowly, but steadily. His hands have grown along with the rest of him, and although his fingers still aren’t especially wide, two of them are significantly more of a challenge to take than one. Eren’s flesh aches and burns as it expands to accept them.

“All right?” Armin asks softly. Eren nods again. The fingers remain unmoving inside him for a moment, then ease out, then ease back in. Armin repeats the movements several times.

Eren notices that the ache has diminished. He feels... full. It’s not pleasurable in the way that having his cock or the outside of his hole stroked is. But it’s Armin who’s filling him, and knowing that is pleasurable, somehow.

Then Armin does something with the tips of his fingers, crooks them and presses forward, and—

“Aaah! _Fuck!_ ”

Eren’s vision clears. He realizes what Armin just did, he’s done that to Armin before, but for some reason he wasn’t expecting it. He was already breathing fast but now he’s gulping air, and his heart is making his entire body rattle. Into the otherwise-silence he hears Armin say rather drily, “I take it you liked that.”

He’s going to kill Armin. Especially if Armin doesn’t hurry up and do that some more. With effort, Eren croaks, “You could say that.” He feels something like a breeze across his forehead and realizes it’s sweat cooling.

Armin’s fingers slide out a little bit. The moment they’re completely reburied in Eren, he crooks the tips again, pushing harder than before. Eren kind of squeals, which would be embarrassing right now, but the embarrassment can wait for later because Armin’s making him feel like he’s got sex nerves everywhere from his lower belly down through his inner thighs and they’re all firing at once. When Armin stops pressing, Eren sags a little against the wall, though he keeps his palms rigidly flat against it.

He feels Armin’s fingers slip out of him again, then hears more rustling of clothes. Suddenly Armin’s hands, the middle and index fingers of the right glistening with oil, press down over Eren’s. They’re not quite as broad, but they’re longer. The hard point of a chin digs into the crown of Eren’s head, a hard shape presses into the small of his back, and the rough weave of Survey Corps trousers chafes his well-beaten ass.

“Much as I’d love to fuck you into the wall,” Armin whispers from just above Eren’s ears, “I don’t think that’s going to work, logistically. So… first, you’re going to take off your shirt, and then, you’re going to get down on all fours.”

Embarrassment suddenly reasserts itself and Eren’s face is once again burning. He wants to just nod, but he can’t with Armin’s chin pressing his head down, so he answers with a hoarse, unsteady “Okay.”

Suddenly there’s no slick warmth over the tops of Eren’s hands, nothing digging into his scalp or back or rubbing against his ass. The air in the room is cool against his heated buttocks. Nervous at the possibility of making Armin wait one more instant than necessary, Eren shrugs out of his shirt and tosses it to the floor about half a meter away from him, then falls to his hands and knees. He braces his forehead against the cool, smooth floor and, though some part of him fears that Armin will resume spanking him, thrusts his ass as far into the air as it’ll go.

He hears a sharp intake of breath behind him, then more rustling, then the soft sound of clothes hitting the floor. When he feels Armin’s warmth against the back of his body again, it’s not trapped behind fabric. He feels the feather-light hairs on Armin’s thighs, the ropy muscles under his skin, the roughness of calluses on the fingertips that are pushing his buttocks apart, and—

“Oh,” Eren says, his voice a little too high and a little too breathy, as he feels _that_ there for the first time and is too stunned with how arousing the sensation is to remember to be nervous.

Armin eases his cock into Eren the same way he did his fingers: slowly and steadily, and not moving at all once it’s all the way in. Armin’s fingertips are tighter than clamps on Eren’s hipbones, and that hurts like hell and Eren doesn’t care because _Armin is inside him_ and holy _fuck_.

“Eren,” Armin says, his voice ragged. “Oh, Holy Sina, _Eren._ ” Hearing Armin lose his ability with words makes Eren’s cock nearly thump against his belly. For several seconds Armin just breathes, harsh and uneven, and then he gasps, “You all right?”

Eren nods hard and manages a shaky “Yeah.”

“Good, because—” The next words come out of Armin in a breathless rush. “God, I can’t stay still, Eren, I have to _move_ —”

With that he pulls all the way out, then immediately begins to slide back in. He’s still moving slowly and gently, though the way his fingers tremble on Eren’s hips suggests that it costs him a great deal of effort. He remains still for about a second this time before he begins to ease out of Eren again. As Armin enters him the third time, Eren closes his eyes and lets himself register the feeling of fullness again. The ache of his body, straining to accommodate Armin, has returned, but as before pleasure bleeds into it, inundates it.

Armin begins to thrust more steadily into him. He releases his grasp on Eren’s right hip to trail his fingers up Eren’s spine, then across his nape. It’s a sensitive spot for all shifters, and when Armin strokes the skin over it, Eren gasps and contracts hard around him involuntarily. Behind and above him he hears a frantic, choked-off _“Ah!”_ , and if he weren’t such a fast healer he’d expect his cockhead to leave a bruise on his abdomen this time. Deliberately now, experimentally, he squeezes Armin’s cock inside him, and Armin manages a patchy string of words: “Oh, _fuck,_ Eren, God, _yes_ —”

Within a few more thrusts Armin is ramming into him, his cock hitting that same spot he was fingering before, hazing Eren’s vision over and, from the fourth rough stroke onward, tearing yelps out of him. Eren’s contractions are spasms now, entirely out of his control, growing more powerful as the pitch of Armin’s moaning rises and vice versa.

Suddenly there’s warm breath against the nape of his neck. As Armin licks and sucks at the skin, the sound of it obscene, he slips his free hand under Eren and closes it around his cock. Eren jolts against his palm as Armin tugs the foreskin back and forth and pushes his still-slick fingertips against the notch on the underside of the head. 

Out of Eren’s mouth comes a keening tangle of vowels vibrating with _m_ ’s and _n_ ’s. The spasms ripple wider and harder through him now, as if Armin’s dropped a rock into the center of him. He gasps, “Armin—I—I’m—” and then his throat is contracting along with the rest of him.

 _“Do it,”_ Armin hisses, his mouth halfway between Eren’s nape and ear. Eren bucks under him, emitting a wrecked sort of howl as he comes hard over Armin’s hand and his own belly. His arms and legs start to shake as he continues to spurt, moaning steadily now, and he can hear the soft _ptt, ptt_ of a few droplets falling off him and striking the floor.

Armin’s mouth falls away as he straightens up behind Eren, grabs his hips again, and drives himself in hard and fast. Eren, sore and hypersensitive, whimpers with each of the half-dozen thrusts before Armin shouts incoherently and begins to come. His thrusts slow down, grow shallower, and finally he stops, panting, with his arms braced around Eren’s waist and his cheek resting on the crown of Eren’s head.

“I… can’t stay like this,” Eren gasps, his arms and legs about to give out now. Armin shifts his weight off Eren onto the floor and pulls him down against him. His chest heaves against Eren’s back. His skin is hot and sweaty, and his groin is sticky with semen. Eren, too limp and fucked out to care, just lies against him. The floor is hard against his side, especially the bruises Armin left on his right hip, but the coolness makes up for it.

After a few minutes of their breathing evening out and the sweat cooling on their skins, Armin whispers into Eren’s hair, “Bed?”

Eren shakes his head. “Not just yet.”

“Can’t walk straight, huh?” Armin sounds immensely pleased with himself.

Eren snorts. “Yeah, like _you_ can?”

Armin chuckles. “We can stagger.”

“Well, we should probably wash up first anyway. We’re all sticky.”

“I hear Levi’s influence there. Two years ago you wouldn’t have cared. But I think I need to lie here a few more minutes before we attempt that.”

Eren closes his eyes. “That works.”

Ten minutes later they pull themselves off the floor, wincing as a few body hairs stick to the mess there. Eren digs in his closet for a washcloth, then kneels to mop up the shallow puddle of sweat and come. Armin pulls out a second washcloth and a towel, which he tosses onto Eren’s bed, then grabs another set of linens for himself, knotting the ends of the towel around his waist. “Meet you in the shower room?” he asks.

“Yeah, that’s fine,” Eren says from the floor.

_Thud. “Ow!”_

Eren lifts his head at the noise. He snickers out loud before he remembers that he shouldn’t.

Armin turns around, rubbing the side of his head with his free hand. Glaring at Eren, he pulls off his towel and throws it and the washcloth onto a clean part of the floor. Then he heads back toward Eren with narrowed eyes.


End file.
